Fuel vapor recovery systems are known that have carbon fuel vapor canisters coupled to a fuel tank for absorbing fuel vapors. The canister also is coupled to an engine intake manifold through an electronically controlled purge valve. Under purge conditions, and when the intake manifold is below atmospheric pressure, the purge valve, and a vent valve which is connected to the canister, are opened to induct fuel vapors from the canister.
Government agencies have mandated periodic on-board vapor leak testing of the fuel vapor recovery system. After a purge, the purge valve and vent valves may be closed so the canister is below atmospheric pressure. During a subsequent vapor leak test, a significant change in canister pressure may indicate a vapor leak.
The inventors herein have recognized that the manifold vacuum assists full closure and seating of the purge valve. A return spring closes the purge valve and an electrically actuated solenoid opens the valve against the spring. The manifold vacuum therefore helps in closing the valve. The inventors have recognized that when the manifold vacuum is not available, such as when the engine is off in a hybrid vehicle or in the stop condition of a vehicle equipped with stop/start, the purge valve may not always be fully seated in the closed position and subsequent vapor leak tests may then provide a false negative. Furthermore, the canister purge valve may be a variable pulse width modulated device, and the valve seating force may vary depending on the duty cycle of the valve prior to completion of the leak test. A variable canister purge valve seating force may cause fuel system leak detection to be unreliable.
In one example of operation, the inventors herein have overcome these and other issues, by closing the purge valve for a first predetermined time, then fully opening the purge valve for a second predetermined time, then fully closing the purge valve and commencing the leak test. The valve opening for the second predetermined time may be brief and cause the valve bounce off the open stop position thereby adding more momentum to the final closing force to provide a good valve seat.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.